1. Field of the Invention
The present invention relates to light-sensitive mixtures useful as negative-working photoresist compositions containing at least one novolak resin and selected 6-acetoxy cyclohexadienone compounds as photosensitizers. Furthermore, the present invention also relates to substrates coated with these light-sensitive mixtures as well as the process of imaging and developing these light sensitive mixtures on these substrates.
2. Description of Related Art
Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of integrated circuits and printed wiring board circuitry. Generally, in these processes, a thin coating or film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits or aluminum or copper plates of printed wiring boards. The coated substrate is then baked to fix the coating onto the substrate. The baked coated surface of the substrate is next subjected to an image-wise exposure of radiation. This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the coated surface of the substrate.
There are two types of photoresist compositions--negative-working and positive-working. When negative-working photoresist compositions are exposed image-wise to radiation, the areas of the resist composition exposed to the radiation become more insoluble to a developer solution (e.g. a cross-linking reaction occurs) while the unexposed areas of the photoresist coating remain relatively soluble to a developing solution. Thus, treatment of an exposed negative-working resist with a developer causes removal of the non-exposed areas of the resist coating and the creation of a negative image in the photoresist coating. On the other hand, when positive-working photoresist compositions are exposed image-wise to radiation, those areas of the resist composition exposed to the radiation become more soluble to the developer solution (e.g. a decomposition reaction occurs) while those areas not exposed remain relatively insoluble to the developer solution. Thus, treatment of an exposed positive-working resist with the developer causes removal of the exposed areas of the resist coating and the creation of a positive image in the photoresist coating.
After this development operation, the now partially unprotected substrate may be treated with a substrate-etchant solution or plasma gases and the like. This etchant solution or plasma gases etches the portion of the substrate where the photoresist coating was removed during development. The areas of the substrate where the photoresist coating still remains are protected and, thus, an etched pattern is created in the substrate material which corresponds to the photomask used for the image-wise exposure of the radiation. Later, the remaining areas of the photoresist coating may be removed during a stripping operation, leaving a clean etched substrate surface.
Positive-working photoresist compositions are currently favored over negative-working resists because the former generally have better resolution capabilities and pattern transfer characteristics.
Photoresist resolution is defined as the smallest feature which the resist composition can transfer from the photomask to the substrate with a high degree of image edge acuity after exposure and development. In many manufacturing applications today, resist resolution on the order of one micron or less are necessary.
In addition, it is generally desirable that the developed photoresist wall profiles be near vertical relative to the substrate. Such demarcations between developed and undeveloped areas of the resist coating translate into accurate pattern transfer of the mask image onto the substrate.
Still further, many current negative photoresist formulations also swell when subjected to development steps, thereby causing image distortion. And, negative photoresists generally require an organic developer solution. The employment of such organic materials creates special handling and disposal problems for the photoresist fabricator.
On the other hand, positive photoresist formulations are not favored for all commercial applications. For example, positive photoresists such as those based on novolak resins and orthonaphthoquinone diazide photosensitizers have certain processing limitations when their imaging is carried out in the deep ultraviolet region of the light spectrum. In this class of positive resists, both ingredients absorb light from the deep ultraviolet region and, thus, the photoresist requires increased input of radiation to compensate for the unwanted light absorptions.
Accordingly, there is a need for a better negative-working photoresist formulation which overcomes the deficiencies of current negative-working photoresists, especially in the area of the deep UV light region where positive-working resists have limitations as to commercialization. The present invention is believed to be an answer to that need.